Car Electrics

I’m having some electrical issues with my S2. Sometimes when I turn the key the car is completely dead. If I try again the car comes back to life. Sometimes while I’m driving along, the Rev Counter needle jumps around and the car begins to run roughly (stutters). Again, if I switch off and start up again, all goes back to normal for a while. I’ve been told that I should start by checking the Earth points and was hoping that someone could tell me where these are.